As a method for making a lithographic printing plate by electrophotography, there is known a technique to make a lithographic plate by forming toner images on a photoconductive layer provided on a support of a lithographic photosensitive printing plate and, then, removing non-image portions of the photoconductive layer using an etchant.
However, the above technique has a disadvantage called side etching, in which an etchant permeates from non-image portions of a photoconductive layer into image portions, or portions of the photoconductive layer located beneath toner images, and removes these image portions and, thereby lowers the reproducibility and stability of images. In other words, characters and fine lines are thinned as side etching proceeds, and when a halftone photograph original is used, gradation reproducibility is deteriorated, making delicate reproduction of highlights or shadows difficult. And in color printing, color reproducibility also becomes insufficient. Another problem of this technique is a fluctuation of etching ability due to variation of activities of an etchant; this affects side etching and make it difficult to manufacture stably printing plates having a prescribed quality. Further, when side etching is controlled to improve such image reproducibility, removal of non-image portions of the photoconductive layer becomes insufficient and, thereby, stains become liable to occur in printing.
Photosensitive lithographic printing plates having a photoconductive layer containing a quinonediazide compound are known in the art by British Pat. No. 996,315, Japanese Pat. O.P.I. Pub. Nos. 90648/1982, 150953/1983, 194467/1985. These photosensitive lithographic printing plates become lithographic printing plates through the steps of forming toner images electrophotographically on the photoconductive layer containing a photoconductor and a quinonediazide compound, exposing the photoconductive layer through the formed toner images, and then etching the non image portions of the photoconductive layer.
However, these conventional techniques, which use a quinonediazide compound in the photoconductive layer, are not necessarily sufficient in side etching prevention and liable to cause the foregoing troubles due to side etching. Particularly, these are insufficient in gradation reproducibility and color reproducibility to provide high quality printing. Further, preparation of printing plates of uniform quality is difficult, and stains become liable to occur if side etching is controlled. Besides the above, these techniques have problems ascribed to insufficient photosensitivities and other electrophotographic properties. When the content of a photoconductor is increased to enhance the electrophotosensitivity, the irradiation time of active light for a quinonediazide compound becomes longer, protracting the processing time or causing stains in printing by unremoved residual at non-image portions of the photoconductive layer.